In view of the numerous studies showing the production and efficient repair of cyclobutyl dimers in normal cells, it has become vital to evaluate the production and repairability of other UV-induced products, which, if inefficiently repaired, could play a large or even major role in cellular UV sensitivity. Progress has been made with a number of areas. The stereoconfigurations of heterodimers of uracil and thymine have been made in order to establish unequivocally such a photoproduct formed in UV-irradiated DNA. The actual isolation and characterization have been carried out with homodimers of cytosine derivatives. Urd(5-5)Urd and dUrd(5-5)dUrd have been prepared by photo-coupling of 5-bromouridine and 5-bromodeoxyuridine, respectively. The mechanism of their formation has also been investigated. Such a knowledge is of importance to the understanding of radiation sensitivity caused by 5-bromouracil in DNA. An anhydro-cyclic nucleoside has been identified as a new class of photoproduct accompanying the photohydration of deoxyuridine in water. BIBLIOGRAPHIC REFERENCES: W. Hauswirth and S.Y. Wang, "Excited State Processes and Solution Conformation of Dipyrimidine Adducts." Photochem. Photobiol. 25, 161 (1977). S. Sasson, S.Y. Wang, and M. Ehrlich, "5,5'-Diuridinyl, A Major Photoproduct from UV-Irradiation of Polynucleotide containing Bromouracil." Photochem. Photobiol. 25, 11 (1977).